Cum play is a very broad term that covers a lot of different fetishes, but in general, it refers to cum swap (snowballing), cum kissing, and even more "exotic" forms like cum on food, cum on glasses, etc...
I hope you now grasped the general idea, but what really matters in an adult game (at least to me) is having a bit of variation, bondage and sex machines are cool but I think it would be a pity to focus an entire game solely on those.
At the end of the day, this your game, and you can do whatever you want with it, but I suggest you open any porn site like Pornhub or similar and take a look at the "categories" sections, who knows, you might find some new ideas for your game